Amplifying system



Patented May 1, 1928.

UNITED STATES.

PATENT? i i lfi-l crrAntnsw. GREEN, or MILLBURN', new JERSEY, ASSIGNORTO 'wns'rnnn ELECTRIC comranzfmoonroanrnn, or new onK, N. Y., A oonronar on or new YORK.

' AMPLIEYING SYSTEHL,

I Application filed July 19 This invention relates to wave amplifying systems, particularly to systems employing. space discharge amplifiers, and more particularly to systems adapted to amplify Waves within a Wide range of frequencies.

a Une object of the invention isto provide an amplifier which offers to Waves of vvidely differing frequencies. impressed upon its mput terminals an impedance that is substantially constant. A secondobiect is to 1ncrease the input impedance of amplifiers to a high valuefora Wide range of impressed frequencies. A. related object isto secure an increaseddegree of amplification as the result ofthe improvement of the input impedance characteristic of the amplifying de; vice. a

A featureofthe invention is afeed back path between the output and the input circuits of an amplifier which includes an im- .pedance element or a network of impedances proportioned in a particular manner sothat substantially all or a predetermined portion of the input energy is furnished. from the output of the amplifierr V It is Well known that the impedance between the input terminals of a space discharge amplifier has the characteristicvariation of a capacitive reactance and that even at low frequencies its magnitude is not great that the shunting effect 1'1'1aybeignored. The common practice of stepping up the voltage of the impressed waves beforeit is applied to the amplifier terminals meets with a limitation due to this property of the discharge tubes, the greatest voltage ratio being completely determined in accordance with well'known principles, by the relative magnitudes of the amplifier input impedance and the impedance of the wave source. The decrease of the input impedance with I increasingfrequency results in a. decreased value of the maximum voltage ratio that can be obtained; consequently, 'a transformer adapted to stepup the voltage most eiiicient 1y at one frequency cannot maintain the same voltage transformation at higher frequencies, and, in fact, is unable to produce even the-lower ratio determined bythe impedances' at the higher frequencies. The diminution of the voltage as compared with the maximum obtainable is due to the sowide frequency range.

ficed in consequence- 1924.. Serial No. 726,929.

called reflection loss resulting. from the mismatching of the impedances by the transformen. As the frequency increases itmay become so great as to completely counter- I balance the gain in the amplifier.

In communicationsystems it is frequently required to. transmit Waves havinw a very For example; the faithful reproduction of music transmitted by telephone currents may require thetrans mission with; uniform efficiency of, all frequencies between 100 cycles per second and 5,000 cycles per second, a ratio of l to 50'; or

again, the operation of a multiplex carrier wave system may require the use of amplb fiers an'iplifying uniformly currents of all frequencies between 3 000 C. P. S; and 30,000 P. S. 'To meet these requirements t has, generally been necessary to-use trans formers havinga low transformation rat'o suited ,to the eiiioient transmission of fro" quencies near to the higher limit the amplification of the loWerfrequencies being sacrilAnother disa'dvantage arising from the va riation of the input impedance of an amplifier is realized When. selective circuits such as filters or transn'iission equalizers are inserted inv the input circuit of the amplifier. Circuitsof this type should be terminated in constant non-reactive resistances in order that their selective properties .i'naybe utilized with the greatest precision. If no input transformer isused the effect of the discharge tube inputimpedance may benegligibly small but'fifa step-up transformer is used,'the effect becomes grcaterandisincreasingly large as the step-up ratio is in-=' creased. This. is due to the Wll known property. of transformers by. which'the secondary load-impedance appears in the primary side transformed according to the square of the voltage transformation ratio.

According. to this invention these difliculties are overcome by furnishing the current required by the amplifier input.impedance fromv the amplified outputfcurrent. This is accomplished by providing afeed-bacl; circuit in which an E. M. F; is generated substantially alike in phase to that impressed on the discharge tube input terminals and in which themagnitude of the current is con ice trolled by an impedance of predetermined value. When the impedance of the feed back circuit is adjusted tothe proper value, the input impedance becomes substantially infinite for a wide range 'of frequencies in so as it affects the, impressed waves and its 'shunting'efl'ect thereon isreduced to' negligible amount.

Ihe invention will be more fully undcrstood from the detailed description and analysis which follows taken in connection with the accompanying drawings of which Fi 1 shows an amplification system embody g the invention;

Q-represents a modified form of the feed-back circuit;

Fig. 3 illustrates a modified form of the invention in which an addition'al'degree of freedom is permitted in the proportioning of the feed back circuit. I

Figs. 4t and 5 show additional methods or furnishing current to the input circuits;

Figs. 6 and 7 disclose the invention applied to amplifiers of the push-pull type; and

Fig. Sis a theoretical diagram for the purpose of illustrating the principles in volved.

To simplify the explanation of the other figures the circuit of Fig. 8 will be firstdiscussed. In this figure only the important impedances and E. M. F. sources of a system involving the invention. are shown, and these in conventional form] -Waves from a source G having an E. M. F. E are transmitted through a line of impedance Z toan impedance Z which is of the proper magnitude and type forterminating the circuit. In parallel with Z is an impedance Z the efiect of which upon the waves from source G it is desired to neutralize. To effect the neutralization a circuit containing a. source F of E. M. F.

E and an impcdanceZ' is connected to the terminals of impedance Z The E. M. F. E. of source F isassumed to have the same frequency as that of source G- and to be of like phase so that the currents from the two sources tend to oppose each other in the section of the line between points C and I). 'Fhe condition that the impedance Z take no current from the source G is the same as the condition that the current I in the branch C. D. be Zero ll'l'GSPGCilVO of the values of the two E. M. F It may be shown that this condition is defined by the following equations 1 2E igEiLm- J1 0Z1+Z4 2Z3 "l Z2 l' Z5 in which Z is the internal impedance of the source F and E. is the fractional part of E that is applied to the terminals of Z The two E. M. F .s being of the same freuency and of like phase the multiplying factor of Z on the right hand side of Equation (2) is a simple numeric. If then by suitable circuit arrangements the E. M. F.E is caused to bear a. constant ratioto E the neutralization may be accomplished by constructing the impedance Z so that its joint impedance with Z is a simple multiple of the impedance Z at all frequencies.

Since the impedance Z is in general fixed by other conditions it may in certain instances be impossible to construct the iinpedance Z to fulfill this condition rigorously. For example Z maybe of the nature of a pure capacity and Z a resistance in which case there is no known structure for Z that will provide at all frequencies a simple numeric ratio of theim'pedance Z Z to the impedance Z If,.however Z is large'and if E is atleast twice as great as IE it will generally be found that the impedance Z is negligibly small.

It is to be noted that the impedance Z need not be a simple impedance element but may consist of atnetwork of elements ofany type, the impedance Z being constructed in the form of a similar network each element of which is related in magnitude to the correspondin element of Z in a constant ratio.

In a space discharge amplifying system involving the invention the E. M. F. impressed on the feed back circuit corresponds to E and being derived from the energy of the amplifier output is readily maintained in the desired relationship as regards phase and magnitude, to the input E. M. F. There beingsubstantially no transfer of energy through the amplifier this E. M. F. may, except for its control as to phase andmagnitude. be considered as originating in a separate source, the internal impedance of which is principally that of the normal load and the space path of the amplifier jointly in parallel. v

In the system of Fig. 1 waves from "the source G are transmitted through a line or other impedance network 1 to the voltage regulating potentiometer 2 and to the input transforn'ier 3 in which they are stepped up in voltage before being impressed upon the input terminals B and C of amplifier 4. Amplifier 4 is of the customary three-electrode space discharge type having included in an evacuated space a cathode 5, an anode 7, and a control electrode 6. In the output circuit of the amplifier the waves are transformed and; reversed in phase in transformer 8 from the secondary winding 9'of which they are delivered to the load impedance 10. From terminal I) of thesecondary winding ,9 the reversed: E. M. F., which by reversal is brought into like phase with the impressed M. F. is fed back to the control electrode terminal B through a the value Ce 4 tion:

condenser 11 of small capacity to furnish the current required by the input impedance of the amplifier.

The factors that enter into the input impedance of a space discharge amplifier are principally the inter-electrode capacities and the effective amplification factor. In addition to these may be included the external capacities between the input terminals, for example, that of the wiring andthe shunt capacity of the input transformer windings.

The input capacities are shown dotted at 12 and 13, 12 representing the capacity between the control electrode and the anode, and 13 representing the total direct capacity between the input terminals. It may be shown that the effective input capacity has given by the following equathat of Fig. 1 using a space discharge ainplifier' of a common type, will be given. For this amplifier, the part of capacity 18 within the discharge tube'itself has a valueof 12.5 micro-microfarads and the part due to the external wiring is generally about 50 micromicrofarads in systems designed to operate 7 at frequencies up to 50,000 cycles per second. The capacity 12 has a value of 6 micro microfarads. Theamplification constant is 6 and the space discharge path has an effective resistance of 5,000 ohms. It will be assumed that the transformer 9 has a oneto one ratioand that impedance 10 has a resistance equal to that of the'space discharge path. The internal and external parts of the output circuit impedance being equal the effective amplification is equal to half the amplification constant. Substitution of these values in Equation (2) gives a Value for C of 86.5 micro-microfarads. 1 At a frequency of 50,000cycles per second the imdischarge path, inwhich the E. M. Flis pro-- duce'd, and the load impedance 10, the transformer 9 inaddition to having unity ratio is assumed to have very high impedance windings and substantially perfect coupling so that its own impedances may. be ignored. In accordance with a well known circuit theorem this part of the'c-ircuit may, with respect to its relationship with they remaining part 11, be replaced by a single branch having an impedance equal to that of the two actual branches in parallel and containing an F. equal to the actual E. M. F. at

the junction terminals of the two branches. I

Applying this theorem to the example chosen the impedance corresponding to Z of Equation (1) is half of the space path resistance or 2500 ohms, and the effective value of the E. M. F. in the feed back circuit is three times the input M. F. The total impedance of the feed back circuit should therefarads.

As already pointed out it is possible to furnish from the amplifier output the current to other impedances shunting the input circuit. In Fig. 2 which represents an alternative form of that part of F ig. 1 above the section line AA an inductance 14 is shown in parallel with the capacity, the purpose being to furnish the magnetizing current of the input transformer. The input transformer isin effect an inductance in parallel with the amplifier capacity and the compoi nent of the input current taken by it is therefore furnished through the inductance 14 in parallel with the capacity that compensates the amplifier capacity. v V The system of 3 is similar to that of Fig. 1, except that the feed back or neutralizing impedance 16, the form of which is not restricted,is connected to a third winding 15 of transformer 8. The reason for this is that the transformer ratio required for the transmission of. maximum energy to the load 10 is not in general. the same as that which will result in largest ratio of the. feed back impedance 16 to the internal impedance of the feed back source. The following analysis indicates the best value of the latter transiiormatmn ratio. It is assumed that the impedance 16 is so great as to constitute an inappreciable load upon the amplifier. Let the resistance of the space path be denoted by R and the transformed impedance of 10 as viewed from the amplifier output terminals be denoted by M R. Ifthe impedance of 10 contains a reactive component, M will he an imaginary or a complex quantity. Further, let the amplification con stant of the discharge tube be denoted by ,u and the voltage ratio of transformer 8 from the primary winding to the feed back winding 15 be denoted by N. In accordance with the circuit theorem already enunciated the effective impedance of the E. hit. F. source in the feedback circuit is equal to and the E. M. h. therein corresponding to an E. hi. F. E impressed upon the amplifier input terminals B and C 1s equal to i he to the maximum value of the ratio is given by the equation 2 1 +M (6) L ,a h l T his value evidently may differ considerably from the required for the maximum energy transn'iission to the load impedance. It follows from Equation 3 that the E. M. F. in the feed back circuit should be twice the E. M. F. impressed upon the amplifier input terminals and the impedance included in the feed back path shouldbe equal to the impedance to be neutralized.

Instead of predetermining the proper magnitude of the feed back impedance by computation from the COGl'llClGIltS of the other elements of the circuit the value may be determined and adjusted by trial. This method is particularly advantageous for determining the small capacity required to neutralize the input capacity of the discharge tube. 'lo permit making the adjustment the condenser it must be variable and a sensi- 'tive' current measuring device should be inserted in series with the secondary wineing of trans former A wave, preferably of relatively high frequency in the transmission range of the amplifier, is impressed up on the input circuit from sour e 1. thereby setting up a counter E. M. F. in the feed back circuit. lhe condenser 11 is then adjusted until the input current as measured by the current indicator has a mii'iimum value, which should be substantially zero. If it is desired to include in the neutralized impedance the capacity of the input transformer windings the adjustment should be repeated with the current measuring device connected directly in series with the transformer primary winding. In addition the impressed Wave should be of the highest frequency it is desired to transmit. To reduce the current in this branch of the circuit to a minimum value it will in general be necessary to increase the capacity of condenser 11. The condition that is reached when the adjustment has been reached is that the inductance of the transformer secondary winding is in resonance with a small residual capacity. Since in practice the transformer resonance usually occurs at a frequency quite low in the transmission range, the condition reached as the result of the adjustment leaves only an extremly small fraction of the capacity eilcctive in the circuit.

In the system of Fig. 4 the reversal of the feed back E. M. F. accomplished by means ofan auxiliary amplifier instead of by a transformer. of the output wave energy from the amplifier 4 is transmitted through condenser 17 and impressed upon the input terminals of the space discharge amplifier 18, the capacity of condenser 17 being large enough to make its impedance negligibly small. The output of amplifier 18 is delivered to a variable potentiometer 19, the adjustable contact of which is con nected to the neutralizing capacity 11 and thence to the input terminal B of the amplifier a. A high resistance 20 connected between the input terminals of the auxiliary amplifier serves as a leakage path for cl that might otherwise accumulate upon the control lectrode. The adjustment of the feed back circuit in this case may be accomplished by varying the setting of the potentiometer whereby the feed-back voltage is adjusted to the proper value for neutralization.

The systems of Figs. 1, 3 and 4 illustrate the application of the invention to a single amplifier supplying energy to a load impedance. In these systems it is obvious from the foregoing analysis that the phase of the feed-back voltage is affected to someextent by the phase angle of the load impedance. To secure the most complete neutralization it is therefore desirable that the load imped ance be nearly non-reactive, or alternatively t iat it be large in comparison with the space path impedance of the amplifier. V

In the system of Fig. 5 the input impedances of amplifiers connected in tandem are neutralized, the phase reversal of the feedback E. M. E being secured by connecting the feed back circuit between the anode of one amplifier and the control electrode of the preceding amplifier. The coupling between the successive amplifiers comprises high im pedance space current supply c0ils23, stopping condensers 24 and leakage path resistances 25. The feed back circuit from the anode of the second amplifier 21 to the input terminal of the first amplifier 4: includes the neutralizing impedance 1'6 and large condenser 17 to isolate the energizluv i lo

ing potential of the amplifier 21. The feed back circuit between amplifiers 22-and 21 is of similar form. The type of coupling used in this system between the successive stages is such that the external impedance of each output circuit is very large in comparison with the internal space path resist ance and in consequence has but a small upon the phase of the feed back E.

p The system of Fig. 6 comprises two space discharge tubes 26 and 27, the input and the output circuits of which are connected together in the well-known push-pull arrangement. The E. M. F. waves from the source 1 being impressed upon the input terminals through the three windingtransformer 28, the two secondary windings of which are accurately balanced to produce equal voltages. The output voltages of the two amplifiers being in opposition, each may be utilized as a source of. E. M. F. for neutralizing the input impedance ofthe other. Two feed-back paths are provided, each extending from the anode of one tube to the control electrode of the other and including the neutralizing impedance 16 and the isolating condenser 17.

The system of Fig. 7 is similar to that of Fig. 6, except that the feed-back E. M. F. is transformed in auto-transformers 29 and 30 in the proper ratio to reduce to a minimum the effect of the discharge tube and load impedances. The operation of the systems of Figs. 6 and 7 is similar with that of the systems shown in the preceding figures and the limitations that were discussed in connection therewith apply also to these systems.

What is claimed is: Y Y Y 1. A wave amplifier comprising at least one space discharge amplifier tube, an input circuit for said amplifier including a wave source, an output circuit for the waves of amplified energy, and means for applying part of" the energy of said amplified waves to neutralize the flow of current from said source to the input terminals of said amplifier. a

2. A wave amplifier for waves of a wide range of frequencies comprising at least one space discharge amplifier tube, an input circuit for said amplifier coupled to a source of waves of a wide range of frequencies by a I circuit, and a feed back circuit connected to.

said input terminals and by means of a amplifying device having a pair of input terminals a circuit connected to said terminals including a wave source, and a feed back circuit connected between said terminals, said feed back circuitbeing adapted to impress on said terminals a feed back voltage substantially equal in phase and magnitude to the voltage impressed by said source.

5."An amplifying system comprising an amplifying device having a pair of input terminals and having a finite input imped ance defined by impedance meansbe tween said terminals, a circuit including a wave source connected to said terminals, and a feed back circuit likewise connected between said terminals, said feed back circuit being adapted to neutralize the current flow from the wave source to the said impedance means.

6. An amplifying system comprising an an'iplifying device, input terminals therefor and a current path between said terminals including impedancemeans defining the input impedance of said device, a circuit including a wave source connected to said terminals, and a feed back circuit likewise connected between said terminals, said feed back circuit being adapted to receive from the amplified waves a voltage in excess ofthat impressed upon the input terminals from the wave source and including impedance means whereby the feed back voltage impressed upon the amplifier input terminals is reduced to substantial equality in phase and magnitude to the voltage impressed from the wave source.

7. A system in accordance with claim 6 in which the impedance means in the feed back circuit is of similar type to the impedance means defining'the input impedance of the amplifying device. v

8. In an amplifying sytsem, a space discharge amplifier having a' cathode and a control electrode constituting input terminals, and impedance means defining the input impedance of the amplifier between said cludingimpedance means having impedance proportioned with respect to the amplifier input impedance whereby the voltages impressed upon the control electrode are substantially equalized in magnitude.

9. In combination, a space discharge amplifier, an input circuit including a wave source connected between the control electrode and the cathode thereof, and means for neutralizing the capacitative input a'dinittance of said amplifier comprising a. feed back circuit connected to the amplifier grid, said feed back circuit being adapted to derive from the amplified waves a voltage in phase with the voltage impressed upon the amplifier control electrode by said source, and including a series capacity, the value of which bears to the amplifier input capacity the same ratio as the voltage impressed by the wave source to the corresponding excess voltage generated in the feed back circuit.

10. An amplifying system comprising a space discharge amplifier, an input circuit connected between the control electrode and the cathode thereof and including a wave source, a feed back circuit including a condenser connected to the amplifier control electrode and coupled to the amplifier space path through a transformer, the windings of said transformer being so poled and the capacity of said condenser being adjusted to such value that the current to the amplifier control electrode from the wave source is reduced substantially to zero.

In Witness whereof, Ihereunto subscribe my name this 17th day of July, A. 1).. 1924.

CHARLES GREEN. 

